1. Field of the Invention
This invention relates to acrylic copolymers having the capability of absorbing ultraviolet and to water-based coating compositions containing the same.
2. Description of the Prior Art
A coating method has been employed for a long time in which a coating material containing an ultraviolet-absorptive agent is applied to a surface of a material or glass in order to shield ultraviolet light or prevent the same from fading and deteriorating the material. Most of such conventional coating materials have been produced by mixing an ultraviolet absorbing agent having a benzotriazole- or hydroxybenzophenone-skeleton with a polymer having the ability of forming a film. However, these ultraviolet-absorbing agents fail to be increased in concentration in the coating material and also result in bleeding from the ultraviolet absorptive layer with a lapse of time, due to poor miscibility with the polymer.
Such inconveniences can be solved by a method in which to synthesize a polymeric compound having a benzotriazole- or hydroxybenzophenone-skeleton which is contributive to ultraviolet absorption, followed by copolymerization of the polymeric compound with a monomer compound. For example, Japanese Laid-Open Patent Publication No. 9-12642 discloses a method in which a polymeric monomer is prepared by chemically bonding benzotriazole or hydroxybenzophenone to acrylic acid or methacrylic acid and then copolymerized.
However, since benzotriazole and hydroxybenzophenone skeletons having no substituents are restricted in the wavelength range of ultraviolet to absorb, it is difficult to absorb all of the ultraviolet light within the range from 390 to 400 nm with the polymeric compound having a simple benzotriazole or hydroxybenzophenone skeleton. It has been found effective to introduce a chlorine atom or a t-butyl group to a benzotriazole skeleton as disclosed in Japanese Laid-Open Patent Publication No. 57-142975 and Japanese Patent Publication No. 4-20913. However, currently it is not an easy operation to incorporate the benzotriazole skeleton having a chlorine atom or a t-butyl group introduced therein into a polymer chain.
Conventional polymers with an ultraviolet absorbing ability have a drawback that they must be dissolved or dispersed in an organic solvent when being applied to a surface of a material or glass, due to their poor solubility in water. However, since such an organic solvent involves problems that it is highly flammable and toxic to the human body, it has been strongly demanded to develop a coating composition having an ultraviolet absorbing ability and requiring no organic disperse medium.
One object of the present invention is to provide an acrylic copolymer containing a comonomer having a benzotriazole skeleton which is high in absorptivity of ultraviolets and wide in absorptive wavelength range thereof and thus has no necessity of concerning about miscibility to the other polymers.
The other object of the present invention is to provide a water-based coating composition obtained by dispersing in water an acrylic copolymer containing an acrylic monomer with a high ultraviolet absorbing ability and a wide wavelength range thereof and a vinyl monomer containing a carboxyl group, as a comonomer.
According to one embodiment of the present invention, there is provided an acrylic copolymer which may be obtained by copolymerizing a monomer mixture containing an acrylic ester and a methacrylic ester in a total amount of 50 percent by mass or more, 1 to 90 percent by mass of which total amount is an ultraviolet absorbing monomer represented by the formula 
wherein R1 is hydrogen or a methyl group, R2 is a halogen atom or hydrogen and R3 is hydrogen or an alkyl group having 1 to 5 carbon atoms. This acrylic copolymer is very effective in production of non-water-based coating composition having the ability of shielding ultraviolet rays.
According to the other embodiment, there is provided an acrylic copolymer which is obtained by modifying the above-described acrylic copolymer to be hydrophilic. The modified copolymer, which is miscible with an aqueous solvent, may be produced by copolymerizing a monomer mixture containing a carboxyl group-containing vinyl monomer in an amount of 0.3 to 0.5 mole per 1 kg of the mixture and further containing an acrylic ester and a methacrylic ester in a total amount of 50 percent by mass or more, 1 to 90 percent by mass of which total amount is an ultraviolet absorbing monomer.
The water-based coating material according to the present invention comprises (A) 3 to 50 percent by mass of an acrylic copolymer derived from a monomer mixture containing an acrylic ester, a methacrylic ester and a carboxyl group-containing vinyl monomer, (B) 20 percent by mass or more of water, (C) 3 percent by mass or more of a hydrophilic solvent and (D) ammonia and/or amine in an amount of 0.5 to 1.1 chemical equivalent weight of the carboxyl groups in the acrylic copolymer; the monomer mixture containing the carboxyl group-containing vinyl monomer in an amount of 0.3 to 5 moles per 1 kg of the mixture and an ultraviolet absorptive monomer represented by formula (1) above in an amount of 1 to 90 percent by mass of the total amount of the acrylic and methacrylic esters in the monomer mixture.